In the chemical treatment of wood chips, or like comminuted cellulosic fibrous material, to produce paper pulp, typically some initial steaming of the chips is done in the chips bin, the chips are discharged from the chips bin (a vertical vessel) utilizing a vibratory discharge mechanism, and then the chips are treated in a horizontal steamer with a horizontal screw moving the chips through the steamer. There have been long recognized problems associated with such equipment, but in the past solutions to those problems which would still result in effective steaming of the chips, have not been forthcoming.
The problems associated with the prior art techniques include the following: (1) In a chips bin, the vibratory discharge mechanism has a tendency to result in "rat holing" of the chips, that is moving a slug of chips through the center of the vessel, while the chips at the wall do not move as effectively. That is, the chips at the walls of the vessel have a tendency to "hang up", while the chips in the center pass through the vessel. (2) The conventional horizontal steamer can only be operated at about 40-60% of capacity, and the horizontal screw is an expensive and high maintenance piece of equipment. Further, the horizontal screw may have a tendency to cause damage to some of the chips.
According to the invention, the problems associated with the conventional systems, such as described above, can be overcome. According to the present invention, a generally vertically disposed steaming vessel is provided, and can be operated about 90% full. The steaming vessel may be used in place of the chips bin, or in place of the horizontal steaming vessel, or both. The vertical vessel according to the invention does not include the undesirable vibratory discharge at the bottom thereof, but rather comprises a plurality of rotating arms with downwardly extending blades, the arms rotated by a concentric generally vertical shaft in the vessel. Steam is introduced into the vessel through a vertical conduit which is concentric with, and surrounds, the shaft. In this way, chips that are in contact with the steam conduit as they move vertically downwardly in the vessel are heated by transfer of heat from the conduit to the chips.
The steam is discharged from the bottom of the conduit and moves upwardly in the vessel to steam the chips. At the bottom the conduit preferably is in the form of a truncated cone, and a slip connection is provided between that cone and the conduit. The cone can either be made to rotate with the shaft and the blades, or the slip connection merely allows some relative movement should it be stressed by the contact with chips, or inadvertent contact with the rotating arms.
The truncated cone at the bottom of the steam tube preferably has maximum horizontal dimensions that are greater than the horizontal dimensions of the chips outlet in the bottom of the vessel. The cone also is vertically spaced from and overlies the chips outlet. In this way chips that are moving downwardly in the center of the vessel are deflected outwardly at the bottom of the vessel, so that they do not have a tendency to merely "rat hole" through the center of the vessel. The rotating arms with scraper blades comprises the only structure for discharging the chips from the vessel, no vibratory structure being necessary.
According to the invention there is also provided a method of steaming comminuted cellulosic fibrous material, such as wood chips. The method comprises the steps of: (a) Feeding material into the top of the vessel, to establish a column of material in the vessel which moves downwardly therein. (b) Supplying steam to the interior of the vessel so that it passes substantially the height of the vessel, and then moves outwardly and upwardly adjacent the bottom of the vessel. (c) Rotating the discharge element at the bottom of the vessel to cause steamed material to be discharged through the discharge opening in the bottom of the vessel. And, (d) preventing material flowing directly from above the discharge opening into the discharge opening by deflecting the material radially outwardly just above the discharge opening, so that it moves into contact with the rotating discharge element and is moved toward the discharge opening thereby.
It is the primary object of the present invention to provide for the simplified, yet effective, steaming of chips prior to the passage of the chips to a pre-impregnation vessel or a digester. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.